The high burden of overweight and obesity in US children and associated poor health outcomes are driving researchers to identify early predictors of obesity so that prevention efforts can be targeted in a timely manner to at-risk children. Infant rapid weight gain (RWG) is thought to be an early growth pattern predictive of subsequent obesity and may be used to identify at-risk children for early intervention. RWG is most commonly defined as an increase in weight-for-age z-score (WAZ) >0.67, which correlates with crossing one major growth chart percentile line. However, the methods currently used to evaluate infant RWG have two significant limitations. First, the recent recommendation to transition from using the 2000 CDC growth references to the new 2006 WHO growth standards raises questions regarding the evaluation and understanding of infant RWG. The new WHO standards are based on longitudinal data from breastfed children, who exhibit different growth trajectories than formula and mixed-fed infants (the primary data source for the CDC references). Consequently, the conclusions that have been drawn up to now regarding infant RWG and its relationship to subsequent obesity may differ with the adoption of the new standards. Second, there is a lack of clarity about when in infancy RWG should be assessed. Although RWG is defined consistently with regard to the magnitude of gain (increase in WAZ of >0.67), the age interval and timing of when RWG is assessed during infancy vary widely from study to study. Including the age interval and timing constraints that best predict future obesity into the definition of infant RWG will ensure the reliability of the measure. Furthermore, given the different shape of the WAZ growth curves during infancy compared to the CDC growth references, it is possible that the current definition of RWG (change in WAZ >0.67) might not provide the best cutoff for RWG in terms of predicting later obesity. We propose to address these gaps by using longitudinal anthropometric data (n=65,506) drawn from the Kaiser Permanente Northwest Health Maintenance Organization. We will compare the prevalence of RWG in the study population using criteria varying both by growth reference (CDC growth references v. WHO growth standards), and by age interval (0-6 months, 6-12 months, or 0-12 months). In addition, we will determine which set of criteria for RWG best predicts obesity at 3 years of age, taking into account covariates and evaluating different cutpoints of change in WAZ to define RWG in the context of the new WHO growth standards. Should this timely research be funded, the findings will aid researchers and clinicians in selecting a reliable measure of infant RWG that is a valid tool for predicting subsequent obesity. This will help researchers and clinicians effectively target early interventions to at-risk children during the critical period of infancy, when the potential for a sustained change in the trajectory of weight gain and a reduction in obesity risk is at its peak. 1